1. Field of the Invention
The invention concerns method and apparatus for longitudinal orientation of thermoplastic film material, especially with a view to the manufacture of crosslaminates of films which have been uniaxially stretched at a relatively low temperature.
2. Description of the Related Art
It is known that the best all-round strength properties in a crosslaminate are obtained by the followings steps of orientation: first a strong almost uniaxial melt-orientation during the drawdown from the extrusion die, or still better an almost uniaxial orientation while the polymer material is semi-molten, and then further orientation at a rather low temperature. “All-round strength properties” here refers to a combination of tensile strength, yield point, tear propagation resistance and puncture resistance. It is difficult to give a satisfactory explanation why this combination of orientation steps is preferable, but it can briefly be said that when the orientation is carried out in these steps, the molecular chains will exhibit a wide spectrum of different degrees of orientation, and those of relatively low orientation will help the film to re-orient instead of splitting, when it is subjected to tearing or puncturing forces.
However, the stretching at low temperatures causes significant problems, e.g. in films which mainly consists of high density polyethylene (HDPE) or isotactic or syndiotactic polypropylene (PP). One side of this problem is that, when a film is longitudinally stretched, it has a high tendency to contract in the transverse direction, at the same time as its thickness is reduced. This tendency is highest when the temperature is low, e.g. between 10-40° C. which is optimum stretching temperature for HDPE and PP, as far as the achieved properties are concerned. The other side of the problem is that, at these low temperatures the material tends to “neck in”, instead of gradually developing the orientation within a reasonably long zone. This means that the stretching must take place between closely spaced stretching rollers or stretching bars, and unless special precautions are taken this will prevent the film from undergoing the needed contraction in the transverse direction.
In U.S. Pat. No. 3,233,029 (Rasmussen), which was published about 40 years ago, a proposal is made for solution of this problem, namely to “anticipate” a substantial part of the transverse contraction to which the film tends, by longitudinal pleating prior to a stretching within one or more short stretching zones, as this is more exactly expressed in the introduction to the present claims. In U.S. Pat. No. 3,233,029, the pleating mechanism described consists of two sets of discs which are mounted spaced apart upon the shafts, one over and one under the film to become pleated, so that discs in one set mesh between the discs in the other set. Thereby the film is forced to form folds or convolutions. It is further disclosed that the film preferably shall pass over a crown-shaped roller adapted to make the stress upon the borders equal to that in the middle of the film. Crown-shaping means that the roller has biggest diameter in its middle, the diameter gradually decreasing towards its ends. Finally it is disclosed that the film preferably is cooled in the stretching zone, which may conveniently be by covering a stretching bar with felt and keeping this felt wet. The water also, by its lubricating action, helps to allow the film the transverse contraction which eliminates the pleats. No pleats remain in the final product.
The inventor managed to make this old invention work with flexibilized HDPE and PP, but only in relatively narrow widths, insufficient for an industrial production e.g. of crosslaminated industrial bags or crosslaminated cover-sheet. When trying to apply the invention to stiffer film, such as film made from plain HDPE or PP, or when trying it on film of greater width, e.g. 1 m wide, the transverse forces applied by the film always caused a transverse stretching of the film in form of thin, longitudinally extending lines. It appears that the principle to apply longitudinal pleating thereby allowing a film transverse contraction during longitudinal stretching, has up to now only been carried out industrially under conditions which also produce transverse stretching and attenuation along narrow longitudinal lines.
GB1,062,936 (Rasmussen) which also was published about 40 years ago, attacks the problems concerning low-temperature stretching of HDPE or isotactic PP film from another angle. This includes subjecting the film to an initial stretching in evenly spaced longitudinal zones by passing the film under tension over a surface corrugated in the direction of movement of the film, the conditions being such that orientation is initiated while the film is passing over the surface in the zones of the film in contact with the raised parts of the corrugations but substantially not in the intervening zones.
It is explained that this produces an orientation in the form or “a substantially regular pattern of shearing lines or microscopic necking-down zones across the whole width of the film”, which “facilitate the subsequent stretching procedure for producing a strong orienting”.
It is further stated in the general description and in the claims that the corrugated surface either is a grooved bar or a cross-fluted roller. However, the specific description only describes the use of a grooved bar, and the patent does not comprise any indication of the dimensions and practical construction of an applicable grooved roller. In the specific description, the grooved bar is mounted close to the rollers which deliver the film.
The example concerns stretching of a 1 m wide film of HDPE to which was added 10% polyisobutylene. This example was now repeated, using a tubular film extruded with blow-up ratio 1:1, and thereby the claimed advantages of this old invention was reconfirmed. However, using a similar film but extruded in blow-up ratio 1.4:1, the orientation became irregular. When further changing to blown film made from plain HDPE, the process acted damaging to the film. It is noted that blow-up ratio 1:1 is not normally used when extruding film, in part because of the needed size of the extrusion die, and in part because of poorer control of the gauge of the film.
A further draw-back should be mentioned, namely that an immense development of frictional heat takes place when stretching is carried out at industrial velocities over a fixed bar. In WO2009/056601 (Rasmussen et al) which had not been published when the present Application was filed, the first above mentioned invention has been carried further. That improvement is characterized in that the reduction of width takes place gradually within a reduction zone no shorter than half of the original film width, this zone being limited by an upstream roller assembly and a downstream roller or roller assembly installed with varying directions of the axis of rotation, this direction forming an angle of 90° with the machine direction at the middle of the film and gradually changing towards its edges to forward the film in converging manner within the reduction zone. Preferably the width reducing zone is formed between two concentrically arranged “banana” rollers, and preferably there are several pairs of mutually intermeshing grooved banana rollers guiding the film through this zone. All of these grooved banana rollers are concentrical with the banana rollers at the upstream and the downstream end of the width reducing zone.
WO2009/056601 also teaches the feature that the pleating can be carried out in several steps with several sets of grooved rollers, whereby the pitch of the grooves in the sets can be mutually different to develop from a coarser to a finer pleating. Under the conditions that the film surfaces are not too frictional and the film not too thick, the process disclosed in WO2009/056601 will normally be sufficient to achieve a fully satisfactory stretching result, even when plain HDPE or PP film are stretched at about room temperature. However, for lamination purposes the film may have a low melting and consequently rather frictional surface layer. This may resist the elimination of the pleats, which should take place by transverse contraction while the film becomes longitudinally oriented. The stretching occurs mainly while the film is dragged over the roller in the upstream end of the stretching zone, and therefore a high friction between the film and this roller can be harmful. To overcome this problem the pleats must in such cases be as fine as practically possible.